Radiohead, the band from Oxford, finally won the West this week with a string of sell-out concerts - the most sweeping conquest of America by a British group since the era of The Beatles.

It was a triumph that always eluded Oasis and was won by Led Zeppelin, The Who and The Stones only after hard years of touring.

But the phenomenon is the unique manner, not the scale, of Radiohead's success - achieved without hit singles, airtime or promotion apart from word-of-mouth communication by fans across a labyrinth of internet websites - the first ever sell-out mega-tour generated by the audience itself.

The concert locations have been innovative too: Radiohead spurned not only the traditional pop promotion apparatus, but also constraints that tie big bands to the usual venue circuit.

As a result, concerts were played across landscapes such as Grant Park, Chicago, a platform between a lake and the most elegant urban skyline in the world, and on Thursday and Friday nights in New York on the bank of the Hudson.

Radiohead mania has engulfed America in strange ways that befit the music. It was not the early, raw rock albums that caught the American imagination, but the bewitched Kid A - probably the least likely album ever to top the US charts - and now its twin, Amnesiac .

This week the group made the cover of Rolling Stone magazine. The New Yorker also assigned its leading writer on music, Alex Ross - who usually concerns himself with Mozart or Bartok - to try and penetrate the tortured irony of Thom Yorke, Radiohead's vocalist and jester-songwriter. Americans now know more about Abingdon School where the band met than almost any other English institution apart from the royal family.

Not only did Radiohead pull huge crowds to hear disquieting music, but the concerts sold out within minutes of tickets going on sale - without a single poster, flyer or newspaper advertisement - entirely due to a buzz on the web.

This is Radiohead's off-stage genius and the Zeitgeist of things to come in the music business that threatens a revolution in the relationship between bands and audiences - and the record companies' place within it.

The term in vogue is 'disintermediation' - cutting out the middle man. 'Radiohead stands on the cusp of something entirely new in the relationship between the producers and consumers of music,' says Andrew Blau, a New York consultant on digital technology.

Radiohead has become a subterranean cyberculture in America, supported by more than 1,000 fan websites. The band's own British-based site simply passes visitors on to 'other places of interest' which know more than it does. But the physical conquest of the US on stage is something entirely new.

After The Beatles' famous ascendancy in America, The Stones, The Who and Led Zeppelin followed with a strategy of touring relentlessly. David Bowie, Pink Floyd and U2 had to stage massive, expensive, loss-making shows to spread the gospel of their records. Oasis was a support band for U2 but, for all the hype, the Gallaghers never broke through.

But Radiohead last week spread its wings across America by doing nothing except play their very cerebral, un-American music.

It all started with Kid A last year, the startling fortunes of which in the US were, says Blau, 'a phenomenon'. Capitol Records in New York made a bold decision to turn the Napster bootlegging case - in which bands and record companies sought to protect music from internet piracy - on its head.

Realising the potential of millions of 'very, very dedicated' fans who were also 'very, very net savvy', Capitol's head of new media, Robin Bechtel, elected to let fans do the marketing and promotion themselves.

Capitol set up its own website, which enthusiasts embraced. The company supplied a special instant 'messaging buddy' as a bridge to unofficial sites, releasing information, music and pictures that fans could easily, and legally, incorporate into their own.

Most controversially, Radiohead and Capitol encouraged fans to copy and circulate free bootlegs of Kid A in its entirety across their own sites three weeks in advance of the album's official release, upon which it went straight to number one with no radio airplay, no video and no hit single.

Amnesiac did the same, reaching number two last June. Even Thom Yorke admits: 'We couldn't understand it.'

This was the propulsion behind Radiohead's sell-out tour of America this week. Capitol took a cue from the band's political distaste for the music industry establishment and played along.

Interviewed by the Chicago Tribune last week, Yorke said he regarded the US tour promotion and ticket sales industry as 'utterly bent, if you ask me', with scathing attacks on venue owners. Yorke added: 'We have done as much as we possibly could to take control and play in outdoor and interesting venues.'

Capitol, meanwhile, simply 'posted details of where and how to get tickets on the web', says Blau. 'So information went direct to the fans, thereby reaching people most likely to be interested and doing it for almost no money, unlike an ad in the New York Times . It's very savvy business and also makes sure the people who are most interested in going get the tickets. The fans participate in the dissemination and marketing of the music - they become the marketing arm.'

Word spread across such sites as Green Plastic - probably the most popular and comprehensive Radiohead fan address - which posted details, corralled the stampede by a passionate core of fans on to sales outlets so that tickets disappeared, on average, within four minutes of going on sale and continued to inform about each concert, playlists, local radio coverage and traffic hazards.

For the moment, Radiohead, Capitol and fans operate a synergy that works for all of them. But Blau says these developments herald changes to come. 'One can imagine a next phase in which a band as big as Radiohead can say "Hang on, why do we need Capitol", and the whole relationship between the musicians and their fans changes,' he says.

'Maybe not every band could, or would, want to do this - but there is a possibility that people as clearly intelligent as Radiohead could now, if they wanted, completely renegotiate their relationship with the industry.'